Conventional high voltage DC sources rely upon inductive coupling of energy from a low voltage AC source to secondary coils which include a rectifier assembly for rectifying the AC voltage to produce a DC output. The increase in output voltage level is achieved by the transformer principle, i.e. the low voltage AC source is connected to a primary coil having a small number of turns and the rectifying components are connected to a secondary coil having a large number of turns. The inductive coupling in the high turns ratio secondary increases or "steps-up" the voltage to the high output level.
The development of high frequency solid state switching devices has made it possible to reduce the size of high voltage DC power supplies through the generation of high frequency alternating voltages for the transformer and rectifier stage. However to operate at frequencies of 100 kHz and above it is necessary to use different geometries than those used at low frequencies. The modified geometries are necessary because the stray, i.e. parasitic, impedances of the high voltage secondary winding are reflected to the low voltage primary winding and thereby to the AC source. The reflected impedances are relevant for high frequency operation because they limit the use of conventional transformers to frequencies of 10 kHz or less.
In the art, Collier (U.S. Pat. No. 5,166,965) has developed a high voltage DC power supply that overcomes some of the problems associated with operating at frequencies of 100 kHz and above. Problems however remain with the power supply disclosed by Collier. First, a high voltage supply as taught by Collier that relies on an increase in the voltage by transformer action will have inherent electrical insulation problems. A high AC voltage can produce partial discharges, or corona, in the voids and defects in the solid insulation material of the secondary winding. These discharges progressively damage the insulation and eventually lead to electrical breakdown. The problem is exacerbated by high frequency operation. As the frequency of operation is increased, the number of discharges per second in the voids or defects increases in proportion to the frequency. Therefore, for operation at frequencies of 100 kHz or higher, solid insulation can experience a very high number of partial discharges making it very susceptible to damage. Furthermore, high voltage AC electrical fields created by high AC voltages on the secondary side lead to dielectric heating of the insulating solids and liquids. This heating can also cause voids in the solids and bubbles in the liquids and also chemical deterioration in both the solid and liquid insulation. These damaging effects arising from high AC voltage on the secondary severely limit the life of high voltage DC sources operating at high frequencies. In order to avoid early failure, it is necessary to employ costly manufacturing techniques and construct insulation with relatively large spacing between the components.
Another problem encountered with conventional high frequency DC power supplies such as taught by Collier is that operating a DC source with a supply frequency of 100 kHz requires the rectification to be carried out with ultra-fast rectifying diodes having a recovery time in the order of a few tens of nanoseconds. Collier makes use of multiple coils each with its own rectifying element, and very high output voltages are produced by adding coil voltages of a few kilovolts each. While Collier's design has the advantage of reducing the rating of the rectifying diodes to a few kilovolts and thereby making the design suitable for implementation, there are still a very limited range of diodes which are suitable for the Collier configuration. It would appear that in fact there is only one type of diode produced by one manufacturer which will allow the Collier power supply to operate as intended. Unfortunately, such diodes are costly, and the single source of such components can jeopardize the commercial viability of such a high voltage DC source.
A third problem encountered with the high voltage DC power supply disclosed by Collier concerns the parasitic impedances which are reflected from the secondary winding into the primary winding. Although Collier uses a relatively small turns ratio, the parasitic impedances of the secondary winding are still significant when reflected into the primary.
The present invention overcomes all of these disadvantages. The present invention comprises a device which produces a high DC output voltage by inductively coupling energy from a low voltage AC source to a series of secondary coils having a low number of turns, in most applications one turn only, and rectifying the AC voltage on each secondary coil to generate a DC voltage. The DC voltages are connected in series to produce the required high voltage DC output.